This invention relates to a gas refrigerator and more particularly to a gas refrigerator such as a Stirling refrigerator wherein refrigeration is provided in one portion of the working space by the reciprocating motion of a displacer.
FIG. 1 is a sectional side view showing the construction of the conventional refrigerator similar to the Stirling cycle gas refrigerator disclosed in U.S. Pat. No. 3,991,585 issued on November 16, 1976. In the FIG. 1 is a cylinder, in which a piston 2 makes a reciprocating motion. 3 is a cold finger having contained therein a reciprocating displacer 4 and connected at its lower portion to the cylinder 1 through a communication pipe 5. The displacer 4 has a working surface 4b at its uppor portion to define an expansion space 6. There are a first compression space 7 defined between a bottom working surface 4a of the displacer 4 and the communication pipe 5, a second compression space 8 defined between an upper working surface 2a of the piston 2 and the communication pipe 5, a space in a regenerator 9 within the displacer 4, and a space in the communication pipe 5. The above-mentioned spaces together difine a working space. The regenerator 9 may be communicated with the expansion space 6 disposed above the regenerator 9 and may be also communicated with the first compression space 7 below the regenerator 9. In the illustrated refrigerator, a freezer 13 is provided as a heat exchanger for the heat-exchange between the expanded, cold working gas and a matter to be cooled. Between the piston 2 and the wall of the cylinder 1, a seal 14 is disposed so that a flow of the working gas between a buffer space 15 defined below the piston 2 and the above-mentioned working space is prevented. Also, the narrow gap between the displacer 4 and the cold finger 3 constitutes a gap seal 16 so that this narrow gap ensures that the flow of the working gas between the expansion space 6 and the first compression space 7 is forced to extend through the regenerator 9.
The piston 2 has disposed within the buffer space 15 below the piston 2 a sleeve 17 made of a non-magnetic and non-magnetizable material such as aluminium. A length of conductor is wound around the sleeve 17 to form a coil 18 which is connected to lead wires 19 and 20 extending through the wall of the cylinder 1 and connected to electrical terminals 21 and 22, respectively, at the outside of the cylinder 1. The coil 18 is allowed to reciprocatingly move in the direction of axis of the piston 2 within an annular gap 23 in which an armature magnetic field is generated. The lines of force of this armature magnetic field extend in the radial direction transversely of the direction of movement of the coil 18. In this case, the permanent magnet is constituted by a ring-shaped magnet field 24 having an upper and a lower magnetic pole, a soft-iron ring-shaped disc 25, a soft-iron cylinder 26 and a soft-iron circular disc 27. The ring-shaped permanent magnet 24, the soft-iron ring-shaped disc 25, the soft-iron cylinder 26 and the soft-iron circular disc 27 together define a closed magnetic circuit or a closed circuit for lines of magnetic force. The above-mentioned sleeve 17, the coil 18, the leads 19, 20, the annular gap 23, the ring-shaped permanent magnet 24, the soft-iron ring-shaped disc 25, the soft-iron cylinder 26 and the soft-iron circular disc 27 together constitute a linear motor 28 for driving the piston. The piston 2 and the displacer 4 are reciprocatingly movably inserted within the cylinder 1 and the cold finger 3 through the piston elastic member 29 and the displacer elastic member 30, respectively, whereby the piston 2 and the displacer 4 are determined as to their fixed position while they are stationary and the neutral position during operation.
The operation of the above-described conventional Stirling cycle gas refrigerator will now be described. When an a.c. source (not shown) having a resonance frequency equal to that of the system is connected across the electrical terminals 21 and 22, a circumferential alternating current flows through the coil 18, so that a periodical axial Lorentz force acts upon the coil 18 due to the alternating current and the radial magnetic field generated by the ring-shaped permanent magnet 24. Therefore, the system which comprises the assembly composed of the piston 2, the sleeve 17 and the coil 18 and the piston elastic member 29 is brought into a resonate state, causing the above assembly to oscillate in the axial direction. The oscillation of the piston 2 causes a periodic pressure change in the working gas filled within the working space composed of the expansion space 6, the first compression space 7, the second compression space 8, the communication pipe 5, the regenerator 9, the central bore 10, the central bore 11, the radial-direction flow duct 12 and the freezer 13. This oscillation of the piston 2 also causes an periodic, axial alternating oscillation of the displacer 4 due to the change in the gas flow rate flowing through the regenerator 9. Thus, the displacer 4 including the regenerator 9 axially and reciprocatingly moves within the cold finger 3 at a frequency equal to that of the piston but at a phase different from that of the piston.
When the piston 2 and the displacer 4 move with a proper phase difference maintained therebetween, the working gas filled within the working space constitutes a thermodynamic cycle known as "the reverse Stirling cycle", generating coldness primarily in the expansion space 6 and the freezer 13. The detail of the reverse Stirling cycle and its principles of generating the coldness are explained in "Cryocoolers" G. Walker, Plenum Press, New York, 1983, pp. 177-123. The brief explanation of its principles will now be made.
The gas within the second compression space 8 compressed by the piston 2 is cooled while it is flowing through the communication pipe 5 and flows into the regenerator 9 through the first compression space 7 and the central bore 10. The working gas is pre-cooled by the coldness which accumulated in the regenerator 9 one half-cycle before and flows into the expansion space 6 through the central bore 11, the radial flow duct 12 and the freezer 13. When most of the working gas reaches the expansion space 6, the expansion of the working gas initiates to generate coldness within the expansion space 6. The working gas then flows through the passage backward while dissipating the coldness into the regenerator 9 to enter into the second working space 8, during which the gas absorbes the external heat in the freezer 13 to cool its exterior. When most of the working gas returns to the second working space 8, the compression starts again, restarting the next cycle. With the above-outlined process, the "Stirling cycle" is completed and coldness is generated.
Since the conventional refrigerator is constructed as above described, when the gap seal 16 is worn by the reciprocating motion of the displacer 4, the direct flow of the working gas between the expansion space 6 maintained at a low temperature and the first compression space 7 at a room temperature increases, degrading the cooling capability, disadvantageously reducing the operating life of the refrigerator. Also, the wear particles formed by wearing of the gap seal 16 clog the flow path of the regenerator 9 or the like to make the refrigerator inoperative, decreasing the reliability.